Persistent infection with hepatitis C virus (HCV) is an important cause of chronic liver disease worldwide. Therefore, the development of vaccines to prevent HCV infection, or at least to prevent progression of chronicity, is a major goal. Virus entry is an attractive target for inhibition of virus infection because the entry machinery is extracellular and it is therefore accessible to antibody. Neutralizing antibodies are a principal component of an effective human immune response to many pathogens which bind to specific epitopes on the envelope proteins of virus, and render them incapable of infecting target cells by a variety of mechanisms. Recombinant E1 and E2 glycoproteins of HCV genotype 1a are currently in use as a candidate vaccine in an ongoing Phase I clinical trial in human volunteers (sponsored by DMID, NIH, at the Saint Louis University Vaccine and Treatment Evaluation Unit), and appear to induce neutralizing antibody. We and others have grown HCV in cell culture and developed pseudotype viruses as models for studies of HCV infection, and examined antibody-mediated virus neutralization. We hypothesize that HCV E1E2 vaccine induced antibodies can have strong neutralizing activity in the presence of serum complement, genotype cross-protective effects, or possibly enhance HCV infectivity. We plan an in-depth investigation into the qualitative nature of humoral immune responses induced during a Phase I clinical trial of the HCV candidate vaccine by addressing the following specific aims: (1) Determine the extent and qualitative nature by which complement enhances viral neutralization mediated by vaccine induced antibodies. (2) Study possible cross-neutralization of other HCV genotypes by vaccine induced antibodies. (3) Investigate the role of immune complex formation in modulation of HCV infectivity. Our proposed complimentary experimental approaches in this vaccinated cohort will help in developing successful vaccine strategies for the intervention of the first crucial step of the HCV life cycle.